Cables for electric power, control and communication lines are run underground in order to protect them from above-ground elements and from the interference and damage they would suffer when installed above the ground or on poles or other structures. Conduits, also called ducts, run underground for such cables should be parallel to each other and spaced apart from each other in a controlled manner in order to minimize any electrical interference. This spacing also acts to dissipate the heat generated by transmission of electric power and electric signals in the cables. In addition, the conduits and cables should be protected, primarily from digging, whether with hand tools or with mechanized equipment, such as backhoes.
A trench may be dug and conduits placed into the trench at a distance from each other. For example, a series of conduits may be placed side-by-side in the bottom of the trench and separated from each other by removable spacers. Once the conduits are placed, the spacers may be removed and all space between the conduits filled with earth, sand or concrete. Thus, the space is filled with thermally conducting but electrically insulating material. If there are to be several vertical layers of conduits, this procedure is very inefficient and time-consuming. In another prior art method, spacers are made with large teeth in the general form of a rake to define spaces between conduits. These spacers may then be used to organize and separate the conduits. However, maintaining vertical separation may be challenging with this method.
It is important to fill all the spaces between the conduits no matter which technique is used to space them apart. It is difficult to accomplish this when the conduits are in several vertical layers. The filler material ideally should be a flowable material, i.e., a material that flows freely downward and sideways in all directions when dispensed into the trench. A more-flowable filler material consists of 50 to 100 lb (about 23 to about 45 kg). Portland cement, 2750 lb (about 1250 kg) of fine sand, and 500 lbs. (about 227 kg) water (maximum) per cubic yard (about 0.765 cubic meters), having a 28-day compressive strength of 50-150 psi (about 0.34 MPa-about 1 MPa). A heavier but still flowable filler material includes a normal weight concrete mix with Portland cement, aggregate having a maximum size of ⅜ inch (about 9-10 mm), and sand and water. The heavier material has an 8 inch (about 203 mm) minimum slump and a 28-day compressive strength of 3000 psi (about 21 MPa). The ability to spread and fill the entire space is needed for good heat transfer and thermal conductivity.
One way to insure even spacing between conduits for power and communications cables is to fabricate banks of ducts which are separated by conduit spacers. The duct banks are then encased in concrete or other material as described above. After the concrete has hydrated or set, cables are pulled through the conduits. The concrete provides a heat transfer medium for conducting heat to the surface, normally the ground surface, and also protects the cables from moisture, rodents and any contractors attempting to dig in the immediate vicinity of the duct bank.
Fabrication of a duct bank typically requires preparing an assembly of conduits and spacers in a trench and then encasing the duct bank in concrete. One method of assembling the spacers is to simply place conduits into bores prepared in a first layer of one or more spacers, and then to place additional spacers and conduit atop the bottom layer. A sturdier assembly may be made by positively locking the conduits into the spacers and by locking the spacers themselves together. Spacers typically do not have easy and reliable ways to interlock to each other.
For example, U.S. Pat. No. 4,601,447, depicts conduit spacers with vertical interlocks made of snap-fit joints, with male snap-fit joints facing downward and female snap-fit joints facing upward. The snap-fits mate when the parts are assembled one-to-another vertically. Molding these snap-fit joints requires very tight tolerances on the tooling if the joints are to work and not interfere with assembly. In addition, the arced portion of the spacer, the portion in which the conduit rests, is relatively narrow and may not provide a sturdy and balanced support for a loaded conduit.
In another example, Snap-Loc spacer model SP4W20-2, made by the Carlon Co., Cleveland, Ohio, U.S.A., has slots well above the bottom of the spacer and tabs near the top of the spacer. The tabs are tapered with the narrow portion facing the feet and with the wide portion on the opposite site. The slots are also tapered; however, they are tapered in the opposite direction, with the wide portion on the feet side and with the narrow portion on the opposite side. This may be a result of the tooling used. Thus, when spacers are assembled together, it is more difficult to make the assembly because the tapers are opposed; once assembled, the resulting joint is loose. Of course, the spacers may be assembled with the tapers made in the same direction; if this done however, the vertical interlocks (feet) will face in opposite directions, and the spacers must be alternated in every layer or tier. This amount of detail is very difficult to accomplish in field situations. Even when this is accomplished, however, the fit is very loose and the assembly is not tight or strong. A five-page brochure on these spacers is included in an Information Disclosure Statement accompanying this patent and is hereby incorporated by reference in its entirety.
What is needed is a better conduit spacer suitable for assembly with matching conduit spacers to form a tight, coherent duct bank. These conduit spacers should be easy to assemble and should not require expensive or hard-to-manufacture tooling with very tight tolerances.